High pressure mercury vapor lamp



E. F. REXER 3,168,668

Feb. 2, 1965 HIGH PRESSURE MERCURY VAPOR LAMP Filed April 6. 1961 INVENTOR.

EDWARD F. REXER BY 53am. M

A TTO/PNEY United States Patent Ofitice 3,168,668 Patented Feb. 2, 1965 3,168,668 HTSH PRESSURE MERCURY VAPOR LAMP Edward F. Rexer, Hopkins, Minn, assignor to Honeywell Inc, a corporation of Delaware Filed Apr. 3, 1961, Ser. No. 100,333 3 Ciaims. (Cl. 313184) The present invention relates broadly to a high pressure mercury vapor discharge lamp and more particularly to an electrode for this system.

High pressure mercury vapor lamps are well known in the art and have found extensive use as a source of intense light from a near point source with most of this emission being in the ultraviolet range. The various elements of high pressure mercury vapor lamps are Well known in the art and consist of a quartz envelope with a refractory conducting material, usually tungsten or thoriated tungsten as electrodes. The envelope is filled with a gas such as xenon and a quantity of mercury metal such that during operation when the mercury is totally vaporized the pressure reaches many atmospheres.

The structure of the electrodes in the prior art basically is as follows. The anode consists of a tungsten or other refractory metal rod with a relatively sharp tip. The cathode consists of a tungsten rod having wound about one extremity a thoriated tungsten winding. In the initial application of voltage, the thoriated tungsten winding on the cathode is heated to a temperature suflicient to freely emit the electrons and an arc through the xenon gas is initiated. During the xenon arc phase of the warmup the winding is heated to incandesoence and the arc fills the entire volume of the-bulb. As the temperature rises more and more mercury evaporates and the arc begins to contract. Finally, when all the mercury has vaporized, the arc has contracted so that it fills only the space between the tips of the electrodes. The winding cools and the voltage requirement decreases.

In these types of devices it has been a common problem that the arc tends to shift during service and the voltage tends to increase. This performance is, of course, undesirable. A further problem has been that of useful life of the lamp. In devices of which I am aware the previous light expectancy was approximately one hundred hours.

The present invention provides a lamp which is superior to that of the previous teachings. Through use of the electrode system of the present invention, a stable arc is produced which does not have the tendency to wander about the tip as noted above. In addition, a greatly increased life has been obtained. Nominal life value is four hundred hours with individual lamps operating for periods as long as eight hundred hours. The invention will be more clearly understood with regard to the following description wherein:

FIGURE 1 is a cathode constructed in accordance with the present invention; and

FIGURE 2 shows the cathode positioned within a completed high pressure mercury vapor lamp.

Referring now to the drawings, in FIGURE 1 there is generally designated a cathode 10, having a main body 11 composed of plain tungsten or 2% thoriated tungsten and a diameter of approximately .080 inch. At the arc end of this electrode is a tapered portion having a coil 12 consisting of a winding of .015 inch diameter thoriated tungsten wire. tungsten rod is a getter 14 of tantalum foil. At the extreme arc end there is a rhenium tip 13 approximately .019 inch in diameter inserted into a hole in the tip of Inserted between the wire and the main the tungsten rod approximately .040 inch deep. The extent of the penetration of the rhenium insert into the tungsten rod does not appear to be critical nor does the extent of its extension beyond the tip of the tungsten rod. However, I have found that the best results are obtained if the rhenium tip does not extend more than .010 inch beyond the tip of the tungsten rod. A tip of osmium or iridium may also be used rather than rhenium with essentially the same results. The surface of the rhenium tip that is exposed to the arc is preferentially rounded to avoid concentrating the are too highly at one point.

A cathode formed in accordance with the above description is then placed into a conventional high pressure mercury vapor lamp such as shown in FJIGURE 2. A fill gas of xenon and an appropriate quantity of mercury is inserted into the tube and the system is then ready for operation. These steps are conventional and are well described in the patent literature.

During operation of the device such as indicated above, I have found that an initial voltage of fifty volts is necessary to initiate arcing. Lamp voltage of approximately twenty volts was found after the preliminary warmup period.

The dimensions indicated in the example are not critical and may be varied over some latitude consistent with the dimensions of the desired arc. These matters are well known and will be obvious to those skilled in the art. From the point of view of cost, it is desirable that the rhenium tip be kept both short and small in diameter.

I claim:

1. A high pressure mercury vapor lamp comprising a sealed envelope containing an ionizable medium including an inert starting gas and mercury in a quantity such that when vaporized the pressure will be greater than one atmosphere, a refractory metal anode, and a cathode comprising a tungsten rod member having a tip of rhenium metal projecting beyond said tungsten and inserted a short distance into said tungsten, a coil of thoriated tungsten wound about said tungsten rod near the arc end thereof, and a tantalum foil inserted between said coil and said tungsten rod.

greater than one atmosphere, a refractory metal anode and a cathode comprising a rod selected from the group consisting of tungsten and thoriated tungsten, said rod member having a rhenium metal tip projecting beyond said rod and inserted a short distance into said rod.

3. A high pressure mercury vapor lamp comprising a sealed envelope containing an ionizable medium including an inert gas and mercury in a quantity such that when vaporized the pressure in said envelope will be greater than one atmosphere, a refractory metal anode and a cathode comprising a rod selected from the group consisting of tungsten and thoriated tungsten, said rod having a tip of metal selected from the group consisting of rhenium, osmium, and iridium projecting beyond said rod and inserted a short distance into said rod.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A HIGH PRESSURE MERCURY VAPOR LAMP COMPRISING A SEALED ENVELOPE CONTAINING AN IONIZABLE MEDIUM INCLUDING AN INERT STARTING GAS AND MERCURY IN A QUANTITY SUCH THAT WHEN VAPORIZED THE PRESSURE WILL BE GREATER THAN ONE ATMOSPHERE, A REFRACTORY METAL ANODE, AND A CATHODE COMPRISING A TUNGSTEN ROD MEMBER HAVING A TIP OF RHENIUM METAL PROJECTING BEYOND SAID TUNGSTEN AND INSERTED A SHORT DISTANCE INTO SAID TUNGSTEN, A COIL OF THORIATED TUNGSTEN WOUND ABOUT SAID TUNGSTEN ROD NEAR THE ARC END THEREOF, AND A TANTALUM FOIL INSERTED BETEEN SAID COIL AND SAID TUNGSTEN ROD. 